In response to the global demand for cleaner energy, this research presents a resourceful and sustainable route for biodiesel production through the design of a novel catalyst derived from two agro-waste materials: chicken eggshells and coconut shell–pith biomass. The CaO/activated carbon (CaO/AC) composite catalysts were synthesized via calcination, carbonization, and wet impregnation, followed by thermal treatment, without the use of surfactants, binders, or toxic chemicals, ensuring an environmentally benign process. Characterization by XRD confirmed the presence of crystalline CaO phases and amorphous carbon. FTIR and SEM analyses further validated the functional group interactions and porous surface morphology, which are critical for catalytic performance. The catalyst was evaluated by transesterifying waste cooking oil (WCO) at varying methanol-to-oil ratios (8:1–10:1), catalyst loadings (6–8 wt%), and reaction temperatures (60–65 °C). The best performance was observed with the C-CCS catalyst (derived from coconut shell), achieving a maximum biodiesel yield of 92.48% at a 10:1 MeOH: oil ratio, 8 wt% catalyst loading, 65 °C, and 90 min. The resulting biodiesel met ASTM D6751 specifications in all key fuel properties, including viscosity, density, flash point, acid value, and moisture content. This research presents a low-cost and scalable synthesis strategy that valorizes dual biowaste streams while producing engine-grade biodiesel. Its operational simplicity and high efficiency make it a strong candidate for industrial biodiesel applications, particularly in resource-constrained settings.